The advent of prostate-specific antigen (PSA) testing in the late 1980s has led to a dramatic rise in prostate cancer (PCa) incidence. This is a significant public health problem, as 35-40% of new prostate cancers have a low risk for progression and are unlikely to benefit from treatment, which translates into more than 75,000 U.S. men each year. In light of evidence indicating overdiagnosis and overtreatment of PCa, there has been increasing interest in active surveillance (AS) as an alternative to the active treatments (AT) of surgery and radiotherapy. AS has been proposed as a legitimate approach to minimizing the harms to quality of life (QOL) and disease-related function due to treatment of low-risk PCa (defined by PSA level < 10, Gleason score < 6, and early stage disease). Although AS is now recommended for low-risk PCa by the National Comprehensive Cancer Network and the American Urological Association, AS is considered to be underutilized among men with low-risk disease. However, there has been very little research examining non-clinical factors that affect use of AS, an important question given the public health implications for the treatment of PCa. Our specific aims are to conduct a longitudinal, prospective study among low-risk PCa patients in which we will: 1. Assess the clinical, physician- and patient-related predictors of a) the initial treatment decision of AS vs. AT and b) remaining on AS vs. switching to an AT. 2. Compare longitudinal changes in patient reported outcomes (QOL, disease specific function, treatment satisfaction) at the baseline, 6-, and 24-month assessments among those who chose AS vs. AT. We will conduct the study within the Kaiser Permanente Northern California (KPNC) HMO, which provides the significant practical advantages of a well-defined population, real-time case ascertainment, comprehensive patient reported data collected pre-treatment, and a readily accessible EMR system. Evidence from the 2010 U.S. Census indicates that 15-21% (N > 41 million) of the insured population is enrolled in a group model HMO similar to KPNC and an additional 3% (N = 8 million) receive care from the VA, indicating that the proposed study will be conducted in a setting that represents a substantial segment of the US healthcare market. We will enroll 1465 newly diagnosed, low-risk PCa patients and conduct a baseline telephone interview within 1 month post-diagnosis and two follow-up assessments at 6- and 24 -months post- baseline. Innovation centers on the inclusion of 3 sources of data: decisional and psychological patient outcomes, physician practice patterns, and biomedical and utilization EMR data, in order to fully assess the factors important to men's treatment decisions and patient reported outcomes. Our study also addresses one of the 5 top priority conditions of the IOM's report on comparative effectiveness research. Finally, the establishment of this cohort will easily lend itself to future comparisons of longer term clinical endpoints, overall health services utilization, and the development of physician- and patient-targeted interventions designed to improve decisions that may ultimately reduce the overtreatment of low-risk PCa.